This invention relates to an improved design of a return to neutral mechanism 10 for use in a variable displacement hydraulic unit such as a pump, a hydrostatic transmission (“HST”) or an integrated hydrostatic transmission (“IHT”) incorporating output gearing and axles, and such devices can be used in a wide variety of uses, including vehicles and industrial applications. The operation of a hydrostatic application such as a pump, HST or IHT is generally known in the art and will not be described in detail herein. For example, the arrangement of an IHT and the operation of the components thereof are generally described in U.S. Pat. Nos. 5,314,387 and 6,122,996, the terms of which are incorporated herein by reference.
In general, an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked through a generally closed circuit, and both consist of a rotatable body with pistons mounted therein. Hydraulic fluid such as oil is maintained in the closed circuit, and the HST generally has a sump or reservoir with which the closed circuit can exchange oil. This sump may be formed by the housing itself.
In a typical arrangement, the pump is usually driven by an external motive source such as pulleys or belts connected to an internal combustion engine. The axial pistons of the pump engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor to drive the motor cylinder block and the motor output shaft. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an IHT, or external thereto. The swash plate is generally controlled by a control arm which is connected via linkage to either a hand control or foot pedal mechanism to control direction and speed.
The pump system is fully reversible in a standard HST. As the pump swash plate is moved, the rotational direction of the motor can be changed. The HST closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure or vacuum side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side and vice versa. This hydraulic circuit can be formed as porting formed within the HST housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example.
A hydraulic pump will also have a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump cylinder block does not create any movement of the hydraulic fluid. The swash plate is in neutral when it is generally perpendicular with respect to the pump pistons.
For safety reasons, and for the convenience of the user, it is preferred to have a return to neutral, or zero displacement, feature which forces the swash plate to its neutral position when no force is being applied to the control arm. Such a feature eliminates unintended movement of the vehicle, and returns the unit to neutral in the event of an accident where the vehicle operator is unable to physically disengage the transmission.